"Today, we are excited to unlock this technology for high-performance games, by teaming up with Epic Games. By leveraging this new JavaScript optimization technology, Mozilla has been able to bring Epic's Unreal Engine 3 to the Web. With this port, developers will soon be able to explore limitless possibilities when it comes to porting their popular gaming titles to the Web."

Which Flash plug-in are you using? Because it requires an extremely recent one to work correctly. That is the major drawback. The multithreading doesn't work well without the correct plug-in, and the single threaded runtime is a lot slower.

I admit, this isn't for everyone. But, it works really well, well enough to run Quake 1 with SDL in an XP virtual machine on a Windows 7 laptop with no lag what so ever. And remember - this isn't a target you have to do a lot of extra work with - the majority of your C and C++ code should work out of the box, so long as you use SDL or one of the other supported frameworks for the graphics. No one is stopping you from porting another tool kit too. That beats javascript every time for me personally. Your mileage might vary.